New Raido schema block: ContributorBlock

[ardc-shorn]

In this discussion, "Contributor" means ContributorBlock

Legacy schema does not require (or support) ContributorBlock.

Raido schema:

• Every raid must have at least one Contributor.
• Contributor must have at least one Position.
• Contributor must have at least one Role.
• There must be at least one Contributor with a Position of Leader.
• There cannot be more than one Contributor with a Position of Leader.

As usual, the app-client support for this field is still prototype-level: it only supports one "Leader" Contributor.
Behaviour is undefined if used to edit a raid with multiple contributors.

This has been deployed to demo environment.
Note that I didn't refresh the DB, viewing any RaidoSchemaV1 raid minted before 2022-12-09 will cause an error because it has no contributors.